Lactic acid bacteria (LAB) are non-mobile, gram-positive, non-spore-forming, micro-aerophilic microorganisms widely explored as starter cultures food industry to enhance the gustatory, nutritional value, imparts appetizing flavour, texture to milk, vegetative, meat foods and prolongs their shelf life. This vast review emphasis various LABs widely explored in the food industry. Herein, we have summarized the classification of LAB strains, their metabolic pathways for biosynthesis of lactic acid, ethanol, acetic acid and demonstrated their application in various food industries for making fermented milk (yoghurt), cheese, beverages, bread, and animal foods. The wide spectrum of LAB-based probiotics, bacteriocins, exopolysaccharides, bio preservative and their relevant benefits towards human health has also been discussed. Moreover, LAB bacteriocins and probiotics in food application may limit the growth of pathogenic, while boosting health immunity. Microbial exopolysaccharides have interesting characteristics for the fermented food industry as new functional foods. Later on, we have discussed the various advancement in metabolic engineering, synthetic biology tools, which have gained considerable interest to elucidate the biosynthetic pathway for tailoring cellular metabolism for high activity.
Food processing industries are growing rapidly to meet the demand for hygienic, more nutritious food for the growing population. Fermented food, beverages are safe and have a great significance in human health for centuries, while improving the shelf life of food products with the promotion of organoleptic characteristics . A variety of raw food products such as meat, soybean, fruits, vegetables, fish and milk has now been fermented industrially with increased biotechnological developments [2, 3]. For example, stinky tofu made from the fermentation of soy milk is a rich source of bioactive compounds, i.e. 4, 7-dihydroxy-isoflavandiol, which have a therapeutic application in the bone improvement and menopausal disorder treatment . Thus, advanced fermentative biorefineries have the in-depth potential for sustainable production broad range of products including fermented vegetative, dairy products, meat industries, bio preservative, probiotics, chemicals, biofuels, medicine, etc.
In view of this, lactic acid bacteria (LAB) are natural microflora that consists of gram-positive, non-spore-forming, non-mobile microorganisms and could grow at varying pH ranging from pH 5.5–5.8 [2, 5, 6]. In general, LAB mostly includes Lactobacillus, Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, Streptococcus, Leuconostoc, Aerococcus, Carnobacterium, Enterococcus, Oenococcus, Tetragonococcus, Vagococcus and Weissella genera’s [3, 7, 8]. These LAB bacteria can be classified either in phylum firmicutes or in various classes Bacilli or Lactobacillus. Naturally, LAB can be involved in various anaerobic fermentation activities, where carbohydrates are initially catabolizing into two molecules of 3-carbon pyruvate producing a net gain of two ATP and two NADH molecules, which is then transformed into lactic acid (LA) by regenerating NAD+ allowing glycolysis to perform to make ATP in reduced-oxygen reaction conditions . Various LAB fermentation metabolites such as lactic acid, acetic acid, ethanol, aroma compounds, bacteriocins, exopolysaccharides and enzymes are nowadays widely explored in food processing industries. Historically, in 1857, Louis Pasteur firstly introduced LA fermentation in foods . Later on, in 1878, Joseph Lister and coworkers further isolated pure bacterium from milk, which results in acidification . A study showed that LA fermentation of autoclaved broccoli could increase the glucosinolates and polyphenolic contents from 55 to ∼ 359 μg g–1 dry weight and 903 to ∼ 3105 μg g–1 dry weight respectively, with enhanced bioactivity and nutritional properties .
Nowadays, LABs are used as commercial starter cultures virtue of acidification, proteolytic, and possess antagonistic, antioxidant, antimicrobial, immunomodulatory properties . For instance, probiotics are certain LABs, (i.e. Lactobacillus strains) that are commercially used for the production of fermented foods [13,14,15]. Lactobacillus casei and Lactobacillus acidophilus LABs are mostly commercially employed for renowned probiotics foods such as BIO®, Actimel®, LC1®, and Yakult® . Probiotics have been demonstrated to reduce gastrointestinal acidity and simultaneously improves the intestinal microbiota with inhibition of undesirable micro-bacterial growth. For example, yoghurt produced from the LAB fermentation of milk is highly enriched in proteins, vitamins, calcium, riboflavin and folate and have more nutritional value. Besides this, certain exopolysaccharides (EPS) could also be produced from probiotic strains Lactobacillus acidophilus, which can reduce the growth of colon cancer when supplemented in appropriate quantity [5, 17]. Therefore, increasing interest in prevention in healthcare in countries China, Japan, and India are major driving factors for encouragement in the production of probiotics. As per Grand View Research Report, global probiotic production is estimated to reach USD 77.09 billion by 2025 with a CAGR of 6.9%. Moreover, with increased biotechnological trends, LABs are capable to produce industrially important metabolites for biochemical, cosmetics, pharmaceutical, flavouring compounds, beverages, milk and meat processing industries. For example, LA produced from the fermentation of carbohydrates can either be adopted as a humectant in leather tanning or can be used in drug encapsulation, targeted drug delivery applications, production of biodegradable polymers, etc. Lactic acid fermentation nowadays is considered as an alternative for the production of a biocompatible polymer, i.e. polylactic acid, suitable for biomedical, tissue repairing, food packaging, bone fixation, drug delivery applications [18,19,20]. A study investigated the effect of that dairy product fermentation using Lactobacillus reuteri BCRC14652 significantly increased adhesion abilities and promote LDH release, which could deconstruct the cell membrane of human colon carcinoma cells HT29 in the gastrointestinal tract . Another study revealed that Lactobacillus acidophilus and Lactobacillu casei could increase the apoptosis-induction activity of cancer drugs such as 5-fluorouracil . Soya bean contains 40–41% protein, 8–24% oil and 35% carbohydrate. Tofu can be made from the fermentation of soymilk using certain L. casei and L. acidophilus bacteria . Studies showed that LAB could reduce the isoflavones glycosides/aglycone . Thus, with growing biotechnological advancement, more and more microbial cultures have been inoculated in the food matrix, which has increased the food properties, antimicrobial activity and prolonged storage . Fermented dairy products enriched in LAB could interact with the gastrointestinal tract with gut microorganisms and contribute to good health.
Therefore, in this article, we provide an overview of various LAB bacterial strains, their classification and their application for various food industries. Herein, a wide spectrum of LAB based probiotics, bacteriocins, exopolysaccharides, bio preservative and their relevant benefits for human health has been discussed. Additionally, this article describes the various metabolic pathway for the production of lactic acid, ethanol, acetic acid. Furthermore, LAB application in various food industries for making fermented milk (yoghurt), cheese, beverages, bread for vegetative, dairy, meat processing industry has also been covered. At last various advanced techniques such as metabolic engineering, synthetic biology has been overviewed to elucidate the biosynthetic pathway for tailoring cellular metabolism for high activity.
Lactic acid bacteria (LAB)
LAB constitute a diverse group of gram-positive, non-spore-forming, rod-shaped anaerobic fermenting bacteria . They mainly belong to cocci, coccobacilli such as Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus and Weissella with low-GC DNA base composition < 53% . In a published report, Daba and Elkhateeb  proposed 25 lactobacillus genera such as Lactobacillus, Acetilactobacillus, Agrilactobacillus, Amylolactobacillus, Apilactobacillus, Bombilactobacillus, Companilactobacillus, Dellaglioa, Fructilactobacillus, Furfurilactobacillus, Holzapfelia, Lacticaseibacillus, Latilactobacillus, Lactiplantibacillus, Lapidilactobacillus, Lentilactobacillus, Levilactobacillus, Ligilactobacillus, Limosilactobacillus, Liquorilactobacillus, Loigolactobacilus, Paralactobacillus, Paucilactobacillus, Schleiferilactobacillus, and Secundilactobacillus. These bacteria can grow at an optimal pH range varying 3.5–10.0 and temperature of 5–45 °C. These LABs are traditionally known for the fermentation of carbohydrates (hexoses and pentoses) into lactic acid or a mixture and classified as homo, the hetero or mixed acid fermentative type depending upon the fermentation pathway and product [28, 29]. LABs are highly promising for current biorefineries and are considered safe to use in dairy industries, meat processing industries . For example, Lactobacillus and Leuconostoc are well-known species of LAB found in most of the vegetative fermentations responsible for lactic acid and acetic acid production .
Sugar metabolism pathways in LAB bacteria
The major pathways for hexose and pentose sugars metabolism are governed by homo or heterofermentative mechanisms as shown in Fig. 1. Various LABs such as Streptococcus lactis, lactobacillus lactis, Streptococcus thermophiles, lactobacillus bulgarius, Pediococcus, enterococcus are known to follow the homofermentative pathway. Wherein, C6 sugars, i.e. glucose is catabolized into LA through Embden–Meyerhof pathway. The metabolic pathway involves the d- or l-lactate dehydrogenase enzyme (aldolases enzymes), which selectivity transformed 1 mol of glucose into two moles of LA and 2 mol of ATP . They are highly preferred in the industrial production of LA in food industries. The stereospecificity of LA depends upon the selected LAB strain. LA produced from LAB fermentation exists as conjugate base lactate either as l-lactate or d-lactate at pH 7.4, which can be obtained either as an optically active form or can be produced as a racemic mixture . Chemical synthesis of LA from the petrochemical route results in a racemic d-, l-LA mixture. However, microbial fermentation could selectively produce d- or l-LA . l-LA is most commonly produced by the l-lactate dehydrogenase enzyme using NAD+ enzyme found in bacteria .
Additionally, LAB fermentation could also produce diacetyl, acetaldehyde and hydrogen peroxide, which contributes to aroma, flavour, antibiotic effect in various foods products. Homolactic fermentation is highly favoured in milk fermentation for souring milk for the production of curd, cheese. A study showed that Lactobacillus spp., a homofermentative bacteria helps in the digestion of lactose in the human digestive tract and improves human health. In a novel approach, Dey and Pal  showed that membrane-integrated hybrid fermentation could selectively produce l-(+) LA using homofermentative Lactobacillus delbruckii (NCIM-2025) strain resulted in a maximin of 12.40 g L−1 h−1 productivity.
On the other side, heterofermentative produce an equimolar amount of LA, ethanol, carbon dioxide and one mole of ATP through the Pentose phosphate pathway . Wherein, pyruvate molecules can be reduced to LA, acetic acid into ethanol and CO2. LAB bacteria could grow readily in most of the food substrate and lowers the pH (3.5–4.5) due to acid formation during the fermentation process . LAB strains such as Leuconostoc mesenteroides, Lactobacillus bifermentoous, Leconopstoc lactis have been well documented for the transformation of hexoses and pentose. Most of the heterofermentative LAB strains (e.g. Lactobacillus plantarum) transform pentose sugars to LA and acetic acid through phosphohexoses pathways yielding 0.6 g LA g–1 pentose carbohydrates . Certain lactobacilli have been known for hydrogen peroxide production through oxidation of reduced NADH by glucose oxidase, which showed the antibiotic effect to other pathogenic microbes. For instance, lactobacillus bacteria produce acid during fermentation may kill other bacterial competitors in buttermilk, yoghurt, hence preserving the degradation of food. Alcohol fermentation is another mostly explored fermentation, where pyruvate is reduced into ethanol via. electron donation from NADH. The mechanistic study revealed that the carboxyl group is initially reduced from pyruvate molecules while releasing CO2 and acetaldehyde. Later in the second step, NADH donates an electron to acetaldehyde and regenerate NAD+. The CO2 produced during heterofermentative fermentation in lactobacilli have a preservation effect in foods.
Classification of lactic acid bacteria
In general, LAB fermentative bacteria can be classified as probiotics, bacteriocins, bio preservatives, exopolysaccharides and are mostly preferred in the various food industry as shown in Fig. 2.
Probiotics are certain groups of non-pathogenic lactic acid microorganisms, i.e. Lactobacillus, Pediococcus, and Bifidobacterium, that are beneficial to the host by colonization’s as summarized in Table 1 . These LABs can be isolated from fermented foods, possess antimicrobial substances and could inhibit the growth of pathogenic microbes, confer health and nutritional benefits to the host . Additionally, probiotics provide nutrition enrichment, stimulating gastrointestinal tract development, bears therapeutic, improve immunity, lowers cholesterol levels and have medical activity for the treatment of respiratory and urogenital disorders (Fig. 3) .
According to a published report, the probiotics market accounted for 46.20 billion USD in 2019 and is expected to increase to 75.90 billion USD by 2026 (Grand view research report, 2019). For example, lactose fermentation produces LA and acetic acid, which increased the pH of the gastrointestinal tract, which lead to inhibit the growth of harmful pathogens and prevent hypercholesterolemia . Streptococcus, Leuconostoc, Pediococcus, Aerococcus, Enterococcus, Vagococcus, Lactobacillus, and Carnobacterium are widely deployed for probiotic applications for fishes, crustaceans . More recently, Jin and his colleagues isolated 114 probiotic LAB strains from raw mare milk. They estimated that Pediococcus pentosaceus, followed by Leuconostoc lactis, Lactobacillus helveticus, Lactobacillus plantarum (n = 6), Lactobacillus kefiri, Lactobacillus curvatus, Lactobacillus paracasei, and Lactococcus garvieae were the most commonly LABs, which showed enhanced hydrophobicity, autoaggregation and coaggregation ability, respectively . In a similar context, Lactobacillus plantarum and Entercoccus faecalis; two potential probiotics were identified from M. rosenbergii, from giant freshwater prawn for aquaculture industries, which showed high tolerance towards variation in pH, temperature, salt and hemolysis activity in in-vitro conditions . Lactobaciullus plantarum is promising culture for the improvement of food flavour and enhance desired metabolites. Kadyan et al.  showed that combined LAB-yeast fermentation using whey. Lactobacillus plantarum MTCC 5690 and dairy yeast Kluyveromyces lactis could improve the antagonistic, antioxidative activity of low alcoholic whey-based probiotic drink against E. coli (28.0 ± 0.41 mm) and S. aureus (16.67 ± 3.86 mm), respectively. Camel milk is known to have therapeutic potential against high medicinal values and could boost the immune system. Sharma et al.  successfully isolated 80 LABs of Lactococcus lactis, Enterococcus lactis and Lactobacillus plantarum from camel milk, which showed strong antimicrobial, adhesion and surface hydrophobicity activity. Similarly, fructophlic LAB, i.e. Lactobacillus kunkeei (12 strains) and Fructobacillus fructossus was isolated from honeybee Apis mellifera gastrointestinal tract demonstrated capacity for biofilm formation and inhibition of pathogens adhesion to the bee gut cells, hence could be deployed for probiotic applications . Xia et al.  isolated 61 strains of LAB from ten fermented rose jams. Further study revealed that, out of 61, five isolates such as P. pentosaceus MP3, P. pentosaceus MP11, P. pentosaceus MP13, P. pentosaceus MP16, and P. pentosaceus MY8 showed good probiotic activity. Besides this, postbiotics are certain soluble low molecular weight metabolites produced from food-grade bacteria during the fermentation process. Postbiotics are superior to probiotics attributes to defined chemical composition, safety, easy storage and use, high pH and temperature tolerance and antimicrobial activity. Presently, Del-Immune V® (Lb. rhamnosus V) from Pure Research Products LLC; CytoFlora® (Lb. casei, Lb. plantarum) from BioRay Inc; Aktoflor C from Solopharm company and Zakofalk from Dr Falk, Germany are some of the commercial postbiotics . Fermented dairy products containing probiotics LABs have demonstrated a protective role against colorectal cancer while inhibiting mutagenic activity, reduces enzyme responsible for carcinogens, mutagens or tumour agents [49, 50]. L. acidophilus and L. casei in low-fat dahi could be effective in delaying hyperglycemia, hyperinsulinemia and reduces diabetic risk .
Biobased, biodegradable, biocompatible films could be produced from microorganisms, such as bacteria’s, yeast, and moulds. Microbial extracellular or exopolysaccharides (EPS) are a special type of high molecular weight biopolymers secretions such as nucleic acids (eDNA and eRNA), proteins, lipids, and sugars. biosynthesized by a wide range of LAB bacteria’s during fermentation and bifidobacterial . EPSs are generally secreted by microorganisms in response to their survival and biotic stress such as temperature, pH, light, intensity, salt concentration, which helps to adapt the microbial cell to survive under extreme environmental conditions . These EPS are either produced intracellularly or extracellularly on the bacterial surface and are covalently bonded or loosely associated with the surface to N-acetyl-muramic acid (Mur-NAc) of peptidoglycan. Bacterial polysaccharides have been explored in food, pharmaceutical, milk industries, as yoghurt culture, Streptococcus thermophilus has been demonstrated to affect the yogurt texture, physical properties and also improves the rheological properties .
Xanthan was initially discovered at NRREL in 1950 in Xanthomonas campestris bacteria’s and was approved by FDA in 1969 for food applications as a gelling agent in dairy and bread processing to improve texture, freeze–thaw stability of frozen foods, thicking of koicem chocolates, etc. [68,69,70]. Besides that, Gellan from Sphingomonas elodea, a high molecular with heteropolysaccharides, curdlan from Alcaligenes faecalis, and dextrans from Leuconostoc spp. are certain commercially explored EPS in Bakery applications. For example, EPS could be used to change milk properties (thickening, stabilizing, gelling, syneresis and rheology augmentation) in yoghurt preparation . Literature documented various LABs such as Fructilactobacillus, Lacticaseibacillus, Lactiplantibacillus, Lactobacillus, Lactococcus, Latilactobacillus, Lentilactobacillus, Leuconostoc, Limosilactobacillus, Pediococcus, Streptococcus, and Weissella, which could effectively produce EPS [5, 72, 73]. Although, EPS does not impart energy to the microorganism, however, it provides microbial adhesion and protective covering to retain microorganism cell integrity in harsh conditions, such as dehydration, drought, osmotic stress and reduces pathogenic attacks. EPS could also contribute to the colonization of natural habitats, biofilm recognitions and protect microbial cellwall from lysozymes, toxic compounds, detergents and provide resistance to antibiotics, bacteriophages, or phagocytosis . LABs derived EPS have putative antimicrobial, antiviral, antioxidant, anti-inflammatory, antitumor, immunomodulatory, and blood cholesterol-lowering, and prebiotic activities, hence suitable for fruits and vegetables fermented food applications .
Biochemical classification exopolysaccharides
Based upon synthetic mechanism and monomers; exopolysaccharides can be categorized as homopolysaccharides (HoPS), one type of monosaccharides and heteropolysaccharides (HePS) consisting of two to eight monomeric units as represented in Fig. 3 . HoPS are polymers of glucose or fructose monomeric units such as dextran’s are α or β-linkages polymer ranging molecular weight of 104–108 Da, are generally produced from Lactobacillus, Weissella Oenococcus, Pediococcus and Streptococcus and Leuconostoc strains. These polysaccharides vary in chain length, sugar moiety, linkage, branching, chemical structure . However, HePS are linear or branched heteropolymers with varying degrees of polymerization and molecular weight (104–108 Da) consisting of α and β linkages in chains . They are mainly consisting of dextrans such as d-glucose, d-galactose, and l-rhamnose secreted by Leuconostoc mesenteroides subsp., mesenteroides and Leuconostoc mesenteroides subsp. dextranicum, Mutans (from Streptococcus mutans and Streptococcus sobrinus); Alternan (from Leuconostoc mesenteroides) and in some cases N-acetylglucosamine, N-acetylgalactosamine, or glucuronic acid repeating units which could be characterized by chemical modification such as acetylation and phosphorylations . Looijesteijn and Hugenholtz  produced EPS from Lactococcus lactis subsp. cremoris NIZO B40 strain using glucose, rhamnose and galactose substrate at optimal condition (25 °C, pH 5.8). Similarly, Pachekrepapol et al.  produced different EPS varying in molar mass, size, and chain length using milk fermentation consisting of glucose and galactose with Streptococcus thermophilus strains. Further study revealed that EPS with 0.14 to 1.61 × 106 g mol–1 molar mass was produced with a low level of rhamnose from Streptococcus thermophilus DGCC 7785, ST-10255, and ST4239 strains at a pH of 4.6 and 8–76.4 mg of glucose equivalents/kg. Zheng et al.  produced two EPS varying in size and molecular weight such as (> 106 Da) and (3.3 × 104 Da) consisting of glucose and mannose at optimal pH conditions. Fructan HoPS could be synthesized from sucrose sugars using fructosyltransferase enzymes and can be categorized as levan-type and inulin-type. Levan mainly consists of β-(2,6)-fructosyl bounded units, having several β-(2,1) side chains are produced from Lactobacillus reuteri. while, inulin comprises ramifications and β-(2,1)-fructosyl linkages, with several β-(2,6) side chains with terminal d-fructopyranosyl residues .
Biosynthesis and application in the food industry
Monomeric sugars (galactose, glucose, mannose, rhamnose, ribose, xylose, fructose, fucose, arabinose) and sugar derivatives (galacturonic acid, glucuronic acid, glucosamine, and galactosamine) are the major precursors for the synthesis of EPS. Biosynthesis of EPS involves four major steps such as sugar transportation, sugar nucleotide synthesis, repeating unit formation followed by polymerization of repeating units formed in cytoplasm’s involving two major enzymes proteins. EPS-specific enzymes such as glycosyl- and acetyltransferases polymerize the monomeric units produced by the first groups of enzymes and release them extracellularly. These EPS-specific enzymes involve specific encoded genes in LAB, typically organized in cluster operon structure form (genophore) in LAB strains and transcribed as a single mRNA . These genes are regulated by the promoter and perform, chain length determination, monomeric unit’s synthesis followed by polymerization of repeating unit. HoPS synthesis is a relatively simple process, involving glycosyl-transferases and fructosyl-transferases enzymes, which synthesize glucosyl monomeric units from glucose and fructose without any energy expenditure . The mechanistic study showed that fructansucrase and glycosyltransferase belonging to glycosyltransferase (GTF, E.C. 2.4x.y) group catalyze the hydrolysis of monomeric units and transfer the residues to growing glucan chains . These enzymes can be further categorized as trans glucosidases (e.g. glucan-synthesizing dextran-sucrases, mutansucrases, and reuteransucrases) and trans fructosidases (e.g. fructan-catalyzing transfructosidases levansucrases and inulosucrases) . α-d-glucans EPS mainly consist of dextrans, reuterans, alternans and dextrans, which can be produced from Lactobacillus, Leuconostoc LAB strains. These include dextrans, mutans, reuterans and alternans respectively. Glucans containing α-(1,3), α-(1,4) linkages are termed as mutants and reuterans. Curdlan us an insoluble HoPS with glucan (β-(1,3) monomeric units and have a molecular weight of 5.3 × 104 to 2 × 106 Da . The majority of EPS produced by LAB are HePS containing 2–8 repeating units of two or more monosaccharides. The polymerization mechanism of HePS is more complex due to heterogeneous structure synthesis. The mechanistic study revealed the involvement of glucose-1-phosphate and fructose-6-phosphate, monomeric units via. glycosyl-transferases enzymes in the cytoplasm . Transportation and polymerization of monomeric sugar units involve Wzx/Wzy pathway by several polymerases’ enzymes. Later on after completion of polymerization, EPS are either secreted out of the microbial cell or loosely adhered to the cell wall making a capsule . Typically, 10–400 mg L–1 of EPS could be produced using different LAB strains under optimized conditions. For example, Mostefaoui and coworkers quantitatively yielded 160–740 mg L–1 of EPS yield for Lactobacillus strains, 126–319 mg L–1 for Streptococcus strains, and 132–134 mg L–1 for Pediococcus strains . In another report, Zheng et al.  yielded 515 mg L–1 of EPS Lactiplantibacillus plantarum at optimized conditions.
LAB-derived EPS holds great potential for current food industries (as a starter, adjunct culture or as a bioactive polymer). For example, Kefiran is one of water-soluble branched glucogalactan HePA, composed of mainly glucose and galactose residues in 1: 1.05 ratio and mostly explored in the dairy beverage industry. A study showed that Lactobacillus kefiranofaciens WT-2BT isolated from kefir grains demonstrated to produce a maximum of 2.5 g L–1 of EPS of 7.6 × 105 g mol–1 molecular weight and 39.9 nm size under PYG10 medium with rice hydrolysate as sugar substrate at pH 5.0 and 33 °C in after 7-day incubation . For food preservative application, a high yield such as 30–50 g L–1 of EPS has been always courage. In this concern, Caggianiello et al.  that Xanthomonas campestris could be adopted for high concentrated HePS, which have beneficial rheology modifier applicability for the food industry, i.e., yoghurt and gluten-free bakery preparations. In another report, Di Cagno et al.  explored Streptococcus thermophilus ST446 (LFC-MWPC-EPS) to produce EPS, which was employed for low-fat Italian Caciotta cheese (~ 0.3%) with the supplementation of 0.5% whey protein, which retained 60.5% of moisture. Fat-reduced cheese with desirable properties could be produced using EPS. For example, low-fat Cheddar cheese was manufactured using Lactococcus lactis ssp. cremoris (DPC6532 and DPC6533) strain as a starter, which predominantly increases cheese yield by 8.17% with a 9.49% moisture increase . Dextran EPS produced from Weisella strains using sucrose as substrate could be used to improve the texture and quality of wheat and gluten-free bread and has been approved by European Commission to be used as an additive for bakery applications . EPS produced from L. sanfranciscensis such as β-(2,6) levan could influence bread texture, softening, gluten content and facilitate water absorption properties by prolonging the shelf life .
Lactic acid bacteria are popular in the production of valuable products such as bacteriocins. Bacteriocins are bioactive nontoxic ribosomally synthesized heterogeneous antimicrobial peptides produced and released extracellularly from certain lactic acid bacteria’s with a narrow killing range for prokaryotes . Presently, LAB based Pediocin Pediococcus acidilactici, Nisin produced by Lactococcus lactis, pediocin and Micocin® comprising of carocyclin A carnobacteriocin BM1, and piscicolin 126 bacteriocins are the only commercial bacteriocins (Table 2), which are approved by FDA in US and Canada for food industries and showed promising broad-spectrum antimicrobial activity to be used as food preservatives, food additives and have many social benefits . Recent studies showed that nisin could effectively inhibit 56 isolates of Streptococcus pneumoniae and 33 isolates of S. aureus for the treatment of dental caries and ulcers . US Food and Drug Administration in 1988 approved Nisin, which could selectively inhibit the growth activity of Clostridium and Listeria spores during cheese formation . Various Nisins isolates (e.g. A, Z, F, Q, H, U, U2, and P) were obtained from lactococci, Streptococcus gallolyticus subsp. S. hyointestinalis, and Pasteurianus strains with continuous biotechnology developments in recent years [96, 97]. Bacteriocins possess antimicrobial properties against pathogenic ad non-pathogenic bacteria’s, with high efficiency and non-toxic, thus has promising application in biotechnological industries, pharmaceutical industries .
Classification of bacteriocins
Bacteriocins are classified based upon molecular mass, chemical structure, thermal stability, mode of action, amino acids residues or antimicrobial activity . They can be classified as Class I, Class II, Class III and Class IV based upon the size and post-translationally modified amino acid residues (Table 3). Primary, bacteriocins can be classified based upon lanthionine-containing lantibiotics as, Class I bacteriocins with < 5 kDa size membrane-active peptides) and the non-lanthionine-containing bacteriocins (Class II; < 10 kDa size and exhibited 100–121 °C stability) [99, 100]. Class II could be further subcategorized into three subclasses as IIa, IIb, IIc based upon N-terminal protein sequences. For instance, Subclass IIa consist of Listeria-active proteins chains containing a specific N-terminal peptide chain sequence such as Tyr-Gly-Asn-Gly-Val-Xaa-Cys-. However, Subclass IIb consists of portion complexes with two proteinaceous peptides chains. Besides that, Subclass IIc comprised of thiol-activated peptides cysteine residues. Class III bacteriocins are made of large macromolecules lipids and peptide chains of > 30 kDa in size and are heat-labile bacteriocins. Class IV consists of complex proteins with different chemicals moieties.
For example, Gratia and coworkers in 1925 firstly introduced Bacteriocin, colicin V, which was named by Jacob et al.  in 1953. Nisin, a type of class I bacteriocins were known as lantibiotic could be produced from Lactococcus lactis demonstrated antibacterial activity against gram-positive bacteria are safe to use in food preservatives . For example, Nisin A could be characterized as a thermostable peptide with a molecular mass < 5 kDa containing 34 lanthionine amino acids core peptides units. Several other natural Nisin could also be produced from Lactococcus lactis (nisin Z, F and Q), some streptococci (nisin H, U, U2 and P), Staphylococcus capitis (nisin J) and Blautia obeum (nisin O) [104, 105]. And can be characterized based upon LAB such as Lactococcus, Lactobacillus, Streptococcus, Staphylococcus, Micrococcus, Pediococcus, Listeria, Bacillus, Clostridium and acid-fast Mycobacterium, including food-borne and non-food-related microorganisms. Joo et al.  demonstrated the therapeutic activity of Nisin for oral cancer and showed that low-concentration (2.5%) could reduce the proliferation cell cycle arrest and apoptosis in head and neck squamous cell carcinoma (HNSCC) following proapoptotic cation transport regulator pathways, without effective normal keratinocytes. In another report, Kamarajan et al.  documented that high content of nisin ZP (95%) could effectively induce apoptosis via a calpain-dependent pathway in HNSCC. These, Class I bacteriocin Nisin has a dual mode of action as shown in Fig. 4, which uses peptidoglycan precursor as a docking molecule and prevent ell wall growth in particular bacteria and promote pore formation driven by a membrane potential Δψ-dependent manner . Additionally, it could bind with lipid II, which could be used as a binding template for designing for novel therapeutic drugs synthesis to control bacterial infections . These can be easily synthesized from food-grade gram-positive and gram-negative LAB and are useful in food preservation.
Bacteriocin varies with protein size, microbial targes, mode of action and immunities mechanisms. Colicins are large proteins pore-forming bacteriocin range in size from 449 to 629 amino acids are mostly studied bacteriocins produced from Gram-Negative Bacteria such as E. coli. Bacteriocins of gram-positive bacteria have different transport mechanism to encode to release bacteriocin toxins . More recently, Li et al.  purified a novel bacteriocin designated as XJS01 was obtained from Lactobacillus salivarius CGMCC2070 LAB strain demonstrated antibacterial and antibiofilm applications against Staphylococcus aureus. Antimicrobial activity and molecular mass composition of XJS01 were 666.31 Da and contain F-S-G-L-A-G-D amino acid, which could inhibit the growth of S. aureus strain 2612:1606BL1486 (S. aureus 26) in chicken meat up to 9.85 μg mL–1 inhibitory concentrations. Similarly, plantaricin bacteriocins LPL-1 and PA-1 could be obtained from Lactobacillus coryniformis showed antibacterial activity against Escherichia coli and S. aureus . The antibacterial studies showed that Plantaricin LPL-1 could significantly reduce the viable cell numbers of L. monocytogenes 54,002 up to 16 μg mL–1 concentration. More recently, Xu and coworkers purified novel bacteriocin 1.0320 from Lactobacillus rhamnosus, which showed a broad antimicrobial activity to destroy the integrity of gram-positive and gram-negative bacterial cell membranes. Further study showed that, bacteriocin 1.0320 exhibits 1–3.3 KDa molecular weight, which has high cell permeability and dissipates the cytoplasmic membrane potential under varying pH gradients and had the 0.072 mg mL–1 of a minimum inhibitory concentration of bacteriocin 1.0320 on E. coli UB1005 .
Lactic acid bacteria starters have been considered as a natural preservative for fermented meat and dairy products . Bacteriocins are certain ribosomally peptides synthesized from Lactobacillus plantarum, Pediococcus acidilactici and Enterococcus faecalis, which exhibits a range of antimicrobial activity . The study revealed that nearly 185 LAB bacteriocins have been reported containing gram-positive and gram-negative bacteria . For example, Nisin A is produced from Lactococcus lactis species, the oldest bacteriocins widely used as a bio preservative from the last 50 years in cheese manufacturing against staphylococci, streptococci, bacilli pathogens. Another report showed that antimicrobial activity of bacteriocins was attributed either due to binding with peptidoglycan or lipid II over pathogenic bacterial cell wall, which cased pore formation with leaching essential minerals from cell wall causing rupturing of bacteria cell .
Application of LAB in various food industries
Fermented beverages and food production
Modern biotechnology has been demonstrated a major contribution in the commercial production of beverage and bread production with flavours enrichment, value addition biopreservation, etc. LAB fermentation of wheat bread could improve its taste, storage quality, palatability and flavour . The country like India, Pakistan, acid-leavened bread (e.g. Idli, dosa, dokola could be produced by lactic acid bacterial fermentation using L. mesenteroides and Streptococcus faecalis bacterial strains. In another report, Batra and Millner isolated Torulopsis candida and Trichosporon pulluans bacterium from idli batter, which attributes to the acidity and gas production for swelling of the bread during fermentation. Rice-noodle, known as khanom-jeen could also be produced from acid fermentation of rice before using Lactobacillus species and Streptococcus strains for 3 days. Besides that, most of the Asian countries produces dietary mungbean starch noodles by acid fermentation involving Lactobacillus casei, L. mensenteroides, L. cellobiosus, and L. fermentum. It has been investigated that lactic acid bacteria resulted in pH drop from 6.0 to 4.0, protecting starch granules from putrefaction and microbial spoilage. Recently, Jiang et al.  showed the fermentation of a mixture of corn floor-wheat bran with Lactobacillus delbrueckii subsp.bulgaricus ATCC BAA-365: Lactobacillus acidophilus ACCC10637, (1:1) could significantly improve the antioxidant property, nutritional value, under solid-state fermentation in vivo for animal feed.
Kimchi is mostly widely preferred traditional Korean fermented food made from green vegetables and seasonings [7, 122]. Lee et al.  investigated that Leuconostoc mesenteroides and Lactobacillus sakei mixed starter culture could improve the flavour of kimchi during cabbage fermentation. They reported mannitol (1393.11 mg/100 g), acetic acid (57.70 mg kg–1), and 1141.90 mg kg–1 of lactic acid using hetero-fermentative production at 15 °C for 72 h thereby reduced the pH by 3.9. Kimchi is a probiotic lactic acid fermented food and predominantly produced using Lactobacillus, Lactococcus, Leuconostoc, and Weissella LAB strains.
Milk Processing industries
Yoghurt is the most popular fermented dairy product produced from fermentation using LAB and have multiple health benefits . LAB bacteria’s such as Lactobacillus acidophilus, bulgaricus, Bifidobacterium, Streptococcus thermophilus, Lactobacillus delbrueckii subsp and Lactobacillus casei are traditionally used as starters to convert milk lactose into organic acids (e.g. acetaldehyde, and diacetyl), glucose and galactose under biochemical transformation process as fermentation hydrolysis and lipolysis . LAB fermentation involves the acidification, coagulation, gasification, destabilization of the protein system in milk, which produces flavour substances such as carbonyl compounds, non-volatile and volatile acids [126, 127]. For example, Caseinate particles in milk start coagulation at pH 5.3–5.2, with complete precipitation at pH 4.7–4.7 . This drop-in pH converts lactose into lactic acid, protein to an amino acid, which enhances the yoghurt flavor.
Yoghurt flavoured bases are used widely in the dairy industry for the enhancement of milk flavors using certain lactic acid bacteria using lipase. For example, Huang and coworkers recently investigated the combination of Streptococcus lactis ACCC 11093 and Lactobacillus casei subsp. rhamnosus 6013, L. acidophilus 1.1878, and L. plantarum DMDL 9010 could enhance the amino acid, volatile acid and ester production with sweet and bitter flavours . Another study revelated that lactulose and inulin, i.e. fructooligosaccharides, galactooligosaccharides or lactulose at 2–4% (w/v) concentration could promote the growth of L. delbrueckii ssp. bulgaricus and Lactobacillus casei, which alter the physicochemical characterization of yoghurt . In a published report, the impact of the addition of hawk tea leaves (Litsea coreana L.) on lactic acid fermentation was evaluated. The study showed that addition of 2 and 4% of hawk tea leaves with S. thermophilus, L. acidophilus, and L. casei significantly increased volatile components in hawk tea-based yoghurts enriched in caprylic aldehyde and acetaldehyde compared to control yoghurt (p < 0.05) . A recent study evaluates the physicochemical and antioxidant activity of LAB fermentation of chickpea yam milk. Fermented milk showed a decline in polysaccharides content. Furthermore, sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) profiles after 24 h fermentation demonstrated the production of small peptides molecules, revealing the protein degradation during fermentation .
Meat processing industry
In recent years, meat consumption has been increased worldwide and reached to103.5 million tons. The global share of beef, chicken and pork production has been reached 61.2, 101.8 and 103.8 million tons, the majority of them are shared by EU China and Unites States (USDA 2021). Meat is a good nutritive substrate and may undergo pathogenic contaminations by Listeria monocytogenes, Escherichia coli, Staphylococcus aureus, Salmonella spp., Campylobacter spp., Yersinia enterocolitica, as well as saprophytic bacteria such as Pseudomonas lead to meat deterioration, spoilage and reduce shelf-life during storage [133, 134]. Morales et al.  found that Pseudomonasfragi, followed by Pseudomonas fluorescens and Pseudomonas lundensis are the dominant spoilage through proteolytic lipolytic and saccharolytic and biosurfactant activities the poultry meat aerobically. Salt spraying, steam pasteurization, irradiation, use of chemical preservatives may reduce spoilage. For example, sodium nitrite has initially used as a preservative in the meat industry, which is nitrosamine and considered carcinogenic. In view of this, the European Union legislative has initiated steps to limit the addition of sodium nitrite for the preservation of meat products . Denmark has stricken the regulation to allow a maximum of 60 mg kg–1, whereas, in Europe, it is 150 mg kg–1. Hence, lactic acid bacteria are now days explored in meat processing to produce fermented sausages . A recent report showed that spraying of 2–6% LA over buffalo meat during packaging could substantially reduce the bacterial count . LAB could promote for protection of meat products by inhibiting the growth of microflora during long storage owing to LAB's extracellular protein substances such as bacteriocins . A study showed that Lactobacillus curvatus, Lactobacillus sakei and Pediococcus acidilactici have been widely employed to increase the health benefits and self-life of raw meat items. Besides, LAB produced lactic acid, acetic acid and bacteriocin, contributing to maintaining moisture texture and color of meat products during storage [17, 140]. A recent study demonstrated that the application of L. plantarum SCH1 could replace the need for a high dose of sodium nitrite (150 mg kg–1) for mechanically segregated chicken batter production, while significant inhibition of E. coli . Therefore, certain LAB, such as Pediococcus spp. could produce EPS, which could improve the texture of meat products. Plasma-activated lactic acid is another advancement to improve the microbiological stability of chicken. In a recent report, Qian et al.  displayed a novel sanitiser for chicken drumsticks with 0.39–1.25 log10 CFU g–1 of lactic acid sprayed using a plasma jet for 40–100 s. Egg yolk mainly consists of low (65%) and high-density lipoprotein (16%) and play an essential role in stabilizing emulsions . A recently published report documented that LAB fermentation of egg yolk for 3–9 h time could change the microbial compositions. The study showed that sulfhydryl group content, surface hydrophobicity of protein was reduced with simultaneous increment in the emulsifying activity ranging from 9.07 to 24.61 m2 g–1 between 3 and 9 h of fermentation . In a recent report, Wen et al.  demonstrated that inoculation with Lactobacillus sakei BL6, Pediococcus acidilactici BP2, and Lactobacillus fermentum BL11 lowered the pH and flavour of beef jerky while inhibiting protein oxidation.
Besides, a new bacterial strain i.e. Bacillus atrophaeus OSY-7LA, isolated from Chinese delicacy demonstrated antagonistic activity for Listeria innocua bacteria . Antimicrobial packaging another way of inhibiting the growth of microorganisms without the direct addition of preservatives. Pediocins are antimicrobial peptides produced from Pediococcus sp. could inhibit the growth of several pathogens. A study showed that 25% and 50% incorporation of Pediocin in cellulose base emulsion Listeria innocuous Salmonella sp. on sliced ham . Prolong storage and natural degradation of food products by microbial contamination are serious international concerns. Literature showed microorganisms such as Moraxella, Enterobacter, Lactobacillus, Leuconostoc, Proteusare Pseudomonas, Acinetobacter, Brochothrix and thermosphacta are considered to spoilage the meat products from last decades, various antimicrobial agents have been used in processing to avoid microbial spoilage. Inorganic phosphates, oxidizers, salts, nitrites and certain organic acids such as benzoic acid have been approved in food preservation by General Standard for Food Additives (GSFA) and Food Safety and Inspection Service (FSIS) for food applications. Accordingly, to EU regulation (EU) No 101/2013, lactic acid could be used to reduce harmful microbial contamination (The European Commission 2013). Although, chemical preservation may drastically reduce the food degradation, however, use of chemical preservatives are highly questionable and leading toxic and carcinogenic effect to human health . Devi and Halami  demonstrated that novel poly(lactic acid) and sawdust biocomposite film is known as Pediocin PA-1/AcH could inhibit the growth of Listeria monocytogenes pathogen on chopped meat food during packaging.
Cheeses and tofu
Cheese is a nutritious dairy product enriched in essential minerals, salt, proteins, vitamins, amino acid, fatty acids primarily manufactured from milk. Ripened cheese is free from lactose and is suitable to reduce high blood pressure and anticarcinogenic properties, which contain conjugated linoleic acid and sphingolipids. Various reports showed that substantial growth in world Cheddar cheese or Emmental-type cheese consumption has been increased in recent years . Various starter LAB strains and nonstarter LABs such as Lactobacillus plantarum, Lactobacillus brevis, Lactococcus lactis ssp. and have now been explored in the cheese manufacturing process for organic acid evolution in cheese . Cheese flavour depends upon the microbial degradation of milk products using LAB during the ripening process. For example, a homo- or heterofermentative lactobacilli LAB strain has mostly used in Cheddar cheese-making in the UK. Cheddar cheese has been matured for 6–9 months ranging from 105 to 107 nonstarter LABs (Lactobacillus paracasei subsp. paracasei and Lb. plantarum) . Similarly, thermophilic microbial cultures of Streptococcus thermophilus and Lactobacillus helveticus has been used as a starter for manufacture Swiss (e.g. Emmental, Gruyère) and Italian cheeses (e.g. Parmigiano Reggiano) at elevated temperature (50–56 °C). These starters result in the production of various organic acids. A study determines the culture-dependent and independent LAB diversity in Tulum cheese, a semi-solid cheese used in Turkey is proved from raw milk fermentation under 12 months ripening period. The study showed that Lactobacillus delbrueckii subsp., bulgaricus, Streptococcus spp., Streptococcus gallolyticus, Streptococcus lutetiensis, and Enterococcus LAB are dominantly changing the texture and flavour of Tulum cheese . A study evaluated that combination of LABs and Saccharomyces cerevisiae could reduce the level of Aflatoxin M1 in 30 days storage and increased the self-life of cheese with varying pH conditions . More recently, Panebianco et al.  showed that Lactobacillus sakei, Lactobacillus plantarum group, Lactobacillus plantarum isolated used in Calabrian Italian cheese could inhibit the growth of pathogenic Listeria monocytogenes, which causes human listeriosis.
Soymilk is a colloidal solution containing β-Conglycinin (7S) and glycinin (11S) proteins accounting for 80% of total protein. Tofu is the most widely consumed soy protein in many East Asian countries. Tofu has been known as soybean curd, enrich in minerals, proteins prepared from hot soy milk with a coagulant as magnesium chloride, calcium sulfate [154, 155]. Nowadays, tofu is made by fermentation of soy milk using glucono-δ-lactone or LABSs with satisfactory flavour and nutrition order. Li et al.  isolated eighteen Lactobacillus spp. and thirty-three yeasts strains of LAB from whey fermentation broth. They found that Lactobacillus plantarum JMC-1 showed improved lactic acid and acetic acid production for tofu-coagulation. Further study showed that lactic acid formed during LAB fermentation leads to a decrease in pH which coagulates soymilk compared to CaSO4 tofu and MgCl2 tofu. The hardness and gel properties of tofu can be adjusted using a combination of Transglutaminase (protein-glutamine γ-glutamyl transferase or TGase) (0–7 U/g) and LAB, which improved the water holding capacity and chewiness by forming intra/ and intermolecular iso-peptide cross-linking in soy protein . Another study showed that Lactobacillus plantarum B1–6 could successfully ferment soy whey and could reduce the pH from 8.38 to 3.98 within 24 h fermentation process. Fermentation enhanced the antioxidant properties, total phenolic and isoflavone aglycone contents of soy whey, which are beneficial for human health . In a similar report, Marazza et al.  documented that, Lactobacillus rhamnosus assisted fermentation of soy milk could increase the antioxidant activity by 71.2%. This increment was attributed to an increase in isoflavone aglycone contents during LAB fermentation. These isoflavones in soymilk have been known to prevent hormone-dependent cancers and have been preferentially used as natural tofu in China. . In view of this, Wu et al.  recently investigated that Lactobacillus plantarum D1031 LAB indues soymilk coagulation at pH 5.8–5.8 and analyzed Isoflavone aglycones, with coprecipitate with proteins. Stinky tofu is another well-known traditional Chinese tofu with a strong odour . In another published report, Chao et al.  isolated 168 lactic acid bacteria’s from fermented brine of tofu. DNS (RAPD) analysis and 16S rDNA sequencing, 136 showed Streptococcus, Lactobacillus, Pediococcus, Enterococcus, Lactococcus, Leuconostoc, and Weissella were the major strains responsible for stinky formation. Soymilk aggregation with LAB could sustainably reduce the chemical demand and environmental pollution.
Biotechnological advancement in LAB fermentation approaches
Genetically modified organisms (GMOs) for Saccharomyces cerevisiae, E. coli and lactic acid bacteria have evolved as a result of recent advances in genetic engineering, system, and molecular biology methods (Fig. 5) [25, 163]. Although, synthetic biology and genome engineering tools are very limited for LAB due to restrictive legislation and poor consumer acceptance [164, 165]. Moreover, GMOs LABs are pioneers in the development of high-throughput genome editing tools for industrial LAB strains for food applications . In view of this, initially, synthetic DNA promoters were successfully incorporated by laboratory evolution and random mutagenesis from Lactococcus lactis expression to improve LAB characteristics for food applications . With that metabolic engineering, exploitation could improve the product yield, productivity and broaden carbon source . Understanding the carbohydrate utilization routes and functional properties of the LAB strain for the target product has become critical . Single-cell genomics sequencing techniques offer the ability to interpret cellular fractions in any cell culture, allowing researchers to evaluate cell heterogeneity and better understand how cells evolve and respond to external stimuli.
Furthermore, as LAB demand for industrially relevant food applications has grown, significant efforts in LAB genetic modification have been conducted, with an emphasis on developing novel LAB to improve the quality of industrial foods. LAB bacteria have been chosen to efficiently secrete recombinant proteins into culture conditions because they are widely recognized as safe (GRAS) species . Lactococcus lactis is a microbe that is widely utilized in food fermentation and has been identified as a promising option for industrial biotechnology genetic and metabolic cell engineering. Genetic control of metabolic flow under central carbon metabolism pathways (e.g., glycolysis, pentose pathway, and citric acid route), which could lead to more robust expression systems for fermented products with better aroma, flavor, and nutrient characteristics. As a result, metabolic engineering components combine system biology and synthetic methodologies to better comprehend the complexity of cellular biotransformation pathways, as well as flux distribution tailoring, redirecting, and analytic approaches . Overexpression of lactate dehydrogenase in pyruvate, for example, might change the metabolic pathway and lead to a specific organic acid (lactic acid, acetic acid, formic acid, and -acetolactate) (Fig. 6).
In metabolic engineering strategies for effective production of diverse food products, inactivation and/or controlled or overexpression of certain genes in Lactococcus lactis homofermentative metabolism can be used. The pyruvate carbon metabolism in Lactococcus lactis has been redesigned through a number of studies. By disrupting lactate dehydrogenase or overexpressing NADH oxidase, the sugar metabolism pathway can be guided to produce -acetolactate. As a result of these efforts, the aldB gene, which codes for α-acetolactate, was found to be capable of producing diacetyl from lactose. Besides, suppression of lactate dehydrogenase in lactococcal cells, pyruvate can be converted into L-alanine. For example, an increased rate of mixed acid LAB fermentation can be directed in case of limited sugars conditions, or high aeration inducing low LDH activity and reducing cofactor NADH. A report documented that, physiological turbulence in L. lactis, overexpression of NADH oxidase, could be accomplished by adjusted by pH with high acetic acid production . Diversion in lactate dehydrogenase enzyme pathway of pyruvate pathway could control the metabolic fluxes in wild-type L. lactic cells through series of mutant strains with LDH activities 1–133% of wild type activity in mixed acid pathways. Thus, the inactivation of the d-lactate dehydrogenase gene by chromosomal integration could lead to a production of l-lactic acid production . Pediococcus acidilactici ZY15 has been known to produce d-LA through glucose and xylose utilization. In view of this, Qiu et al.  demonstrated that overexpression of short-chain dehydrogenase through the incorporation of the CGS9114_RS09725 gene could selectively improve the lactic acid production by reducing the inhibitory effect of vanillin. Compared to random mutagenesis; directed DNA medication via. chromosomal gene transfer could lead to activation or inactivation of specific enzymes. In view of this, various techniques such as systematic cloning, chromosomal modification and gene expression, electroporation, vectors etc. have been developed allowing genetically modified LAB. A study reported the biosynthetic model and complete genome sequence of Lactobacillus fermentum YL-11 LAB from fermented milk based upon the bioinformatic analysis of the complete genome. Exopolysaccharides released from YL-11 strain was enriched in galactose (49.2%), glucose (31.1%) .
Genome-scale metabolic models reconstruction can be beneficial for rational metabolic engineering. Kristjansdottir et al.  reconstructed a genome model of L. reuteri JCM 1112 through flux distribution of glycolytic, phosphoketolase and Embden-Meyerhof-Parnas pathways. They performed extensive manual curation such as gap fillings, updating and gene-protein reaction associations, gene IDs update and updating reacting metabolites and reviewed the integration of organism-specific data. The accuracy of models was further validated by experimental results and predicted effects of glycerol addition and adhE gene knock-out impaired ethanol productions. Variety of gene expression and protein targeting expression systems such as NIsin-Controlled gene Expression system (NICE) have been evolved for industrial food applications are associated with plasmids and bacteriophages has been mostly developed for L. plantarum, L. reuteri, L. brevis as well for L. plantarum for antimicrobial applications [178,179,180]. Gene expression system is initially developed first tools induced by bacteriocins and pheromones nisin for food applications. This system has been used for several lactobacilli for pSIP, which uses sakacin-inducer peptides to lowers the background expression . In pSIP, expression of particular gens is under control of strong, inducible bacteriocin promoter and induced by the addition of external peptide pheromones as demonstrated in Lactobacillus spp. based upon the promoter and regularity genes for the overproduction of β-glucuronidase and aminopeptidase for milk processing industries. For example, a report documenting the overexpression of two recombinant genes lacL and lacLM encoding were overexpressed and optimized for heterodimeric β-galactosidase (130 U/mg protein) from Lactobacillus reuteri were overexpressed in host strain potential for enzymes and proteins . These β-galactosidase are helpful in the catalysis of transgalactosylation reactions, transfer galactosyl moieties of lactose into galactooligosaccharides for prebiotic applications.
Genetic manipulation is a key method to visualize the metabolic mechanism of LBA and magnify the production performance of strains and enables it to grow, propagate, and allow to make a stable structure concerning the external atmosphere. Thus, electro transformation or electroporation has been widely employed to increase the transformation of LAB by electric discharge. This creates a temporary pore in the cell wall and enables exogeneous DNA to enter cells through these pores for short period for genetic manipulation. Currently, novel recombinant DNA technology approaches such as prophage recombinases-mediated and CRISPR/Cas9-assisted recombineering has been employed through electroporation . Various factors such as characteristics of the strains, exogeneous plasmids, parameters of electric pulse and weakening agents, washing buffers, culture, heat treatment can significantly increase the electroporation efficiency . Electroporation-assisted single-stranded DNA (ssDNA) recombineering has shown immense potential for point mutations in various lactic acid bacteria. For example, Zhou et al.  established the single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) recombinant CRISPR/Cas9-assisted seamless genome editing in Lactobacillus plantarum through electroporation. These studies were used to improve the single-stranded DNA recombinant efficiency to engineer L. plantarum for N-acetylglucosamine (GlcNAc) productions. The modified strains demonstrated to produce 797.3 mg L–1 of GlcNAc without the introduction of exogenous genes or plasmids. In a similar report, Guo et al.  made Lactococcus lactis NZ9000 recombinant strain carrying plasmid pL-RecT through electrocompetent cells. The study revealed that electroporation of control plasmid pTLCas9 generated 2.72 × 105 CFU. 100 μg–1 plasmid DNA, while transformants yielded 80.5, 56.0, 46.0 and 55.5 plasmids for the four locations respectively. In a similar approach, Landete et al.  showed an efficient way for genetic transformation of LAB strains by optimizing the applied voltage. The result showed an improved transformation efficiency for L. lactis subsp. cremoris MG136 compared to others. Recent multi-pulse electroporation could increase the transformation efficiency compared to single pule electroporation. Welker et al.  Investigated that multi-pulse electroporation could increase eightfold transformation efficiency of Lactococcus lactis and Lactobacillus casei cells resulting in 1.2 × 106 colony forming units (CFU) µg–1 pGH13 compared to single pulse electroporation. In their previous report, they demonstrated that the addition of 1% glycine or 0.9 M NaCl during the growth phase, limits the growth of cell culture. Further study showed that pre-electroporation treatment of cells with water or lithium acetate/ dithiothreitol under electroporation conditions could efficiently increase 106 colony forming units μg−1 pTRKH2 DNA . Sato’o et al.  optimized the multi-phase electroporation for Staphylococcus epidermidis JMUB20 resulted in a 3.3-fold increase in transformation efficiency by1.38 × 103 colony forming units (CFU) µg–1 with an efficiency of 1400 CFU μg–1 of plasmid DNA.
LAB genomes are attributed with small sizes ranging from 1.23 Mb (Lactobacillus sanfranciscensis) to 4.91 Mb (Lactobacillus parakefiri), nearly > 200 LAB strains have been developed through reducing the evolution of particular genes and limiting particular biosynthetic pathways varying in phenotypes. LAB possess several genes, which enables survival under limiting situation of nutrient, pH, salts, temperature and inhibits pathogen growth. Gene mobilization strategies through DNA transfer are a significant step towards any genome editing and can be achieved through natural or artificial methods as shown in Fig. 7. DNA transfer including natural competence, conjugation, and phage transduction is a critical step for genome editing to achieve non-GMO LAB strains using CRISPR-Cas technology . For example, CRISPR-Cas approaches were performed for the construction of tailored genome editing tools for Streptococcus pyogenes type-II Cas9 endonuclease (SpyCas9) . System biology tools may integrate genetics, metabolism information’s for the construction of ideal organism models. A study showed various, kinetic, static, and genome-based models have been collected using metabolic engineering experiments for Lactococcus lactis MG1363 LAB strain . Natural methods employed conjugation approaches, wherein conjugative plasmids and transposons are included in LABs DNA fragments through CRISPR-Cas single-step genome editing . CRISPR-Cas9 technology has been widely exploited for the insertion/ deletion of the genetic element from a range of prokaryotes and eukaryotes . A study demonstrated that bacterial adaptive immune system through CRISPR-Cas system, wherein double-strand DNA breaks and exploited in curing of mobile genetic elements like plasmids, ICSs and genomic islands from various LAB bacteria such as Lactococcus lactis, beneficial for dairy fermentation. They described that Cas9 expression vector pLABTarget, encoding the Streptocccus pyogenes Cas9 under controls promoters allows the introduction of sgRNA sequence to specific genetic loci. Additionally, the introduction of pepN-targeting derivative of pLABTarget into L. lactis strain MG1363 led to a strong reduction in the number of transformants obtained . CRISPRs-cas genes stranded DNA recombineering through genome sequence provides immunity against foreign DNA and complex environment . Certain gene expression systems, specific gene regulatory, synthetic inducible promotors as well as chromosomal insertion systems have been investigated for L. lactis. Genome-scale flux models can e useful to understand the catabolic metabolism of lactic acid bacteria. In view of this, Oliveira et al.  reconstructed the metabolic network of L. lactis ssp. IL1403 using annotated genome sequence and established 621 reactions, physiology and regulatory mechanisms for 509 metabolites. Later on, an in silico metabolic level comparison of the above three bacteria showed that biomass composition was subjected to large species-dependent variation. Wherein, CO2 was identified as potential growth stagnation in anaerobic culture of Lb. plantarum and extended genome analysis predicted the ability to grow over glycerol. In another study, the metabolic pathway for butanol production from glucose was reconstructed through gene encoding of crt, bcd, etfB, etfA, hbd, bcs-operon, and thl gene to lowers clostridia butanol pathway. The resulted pathway was transferred to L. brevis, which yielded 4.1 mM of butanol from a glucose-containing medium . A study documented the genome editing in Lactobacillus crispatus having 5′-AAA-3′ protospacer adjacent motif (PAM) and a 61-nucleotide CRISPR RNA was reconstructed using endogenous type 1 CRISPER-Cas systems . In a report, a kinetic model was developed for pyruvate distribution in Lactococcus lactis NZ9000 by selecting enzyme kinetics parameters and metabolic controls to raise the flux coefficients of metabolites (e.g. acetoin, diacetyl etc.) using reversible Michaelis–Menten equations. The model predicted the knocking of lactate dehydrogenase by overexpressing NADH-oxidase and increased acetolactate synthase from 0 to 75% for targeted products release . Certain attempts have been made for controlled genetic improvement using insertion sequence techniques for bacteriophages resistance, nutrition and flavour enrichment in daily products. A published report showed that deletion of Lac Z gene in lactobacillus bulgaricus limits the lactose fermentation, hence prevents souring of yoghurt . Pathway redirection involves the genetic modification to eliminate competing pathways for pyruvate utilization attribute to the accumulation of undesired metabolites. Reconstruction of glycolysis pathway with glucose transport mechanism and fermentation shift has been extensively studied for Lactococcus lactis spp. lactis . demonstrated that the control of flux through the glycolysis pathways could be regulated by the ATP demands and glucose transport capacity through the allosteric characteristics of gluycolytic enzymes specifically phosphofructokinase, pyruvate formate-lyase, pyruvate kinase, l-lactate dehydrogenase with reduced glycolytic flux. Homolactic fermentation model in Lactococcus lactis LABs may lead to > 90% conversion of sugars into lactic acid with high metabolite rates attribute to high activity of lactate dehydrogenase (LDH), catalyzed by accumulation of NADH formed during glycolysis. However, under anaerobic fermentation conditions, this phenomenon is lost leading in the co-production of acetate, ethanol, formate, CO2 with the reduction in overall conversion yield . This mixed acid metabolism has lower growth rates and occurs under carbon-stress conditions. Therefore, various metabolic models have been developed to control the homolytic shift to mixed acid metabolism controlling the activity of fructose-1,6-bisphosphate, activating the lactate dehydrogenase and of glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate, inhibiting the pyruvate formate lyase enzymes. In view of this, Garrigues et al.  studied the correlation of growth kinetics, metabolism type in two Lactococcus lactis ssp. strains under batch fermentation on lactose and developed a model of glycolysis regulation. They investigated that NADH/NAD+ ratio was the major regulator factor to control the catabolic flux to inhibit the glyceraldehyde-3-phosphate dehydrogenase activity. The controlled flux led to enhanced glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate enzyme concentration while inhibiting the pyruvate formate lyase activity leading to homolactic metabolism . Lactate dehydrogenase inactivation and the direct oxidation of NADH via. oxygen may result in the production of a mixture of fermented catabolites such as acetic acid and acetoin. Homofermentative lactobacillus delbrueckii subsp. bulgaricus and L. delbrueckii subsp. lactis has been extensively used in the dairy industry. After consecutive mutations, these strains may lose ability to regulate lac operon expression. The sequence analysis of lactose operon in bulgaricus and delbrueckii subsp. lactic were discovered, regulated and inserted complex IS element (ISL4 inside ISL5) in lac promotor of both strains to enhance the activity . In a comparative report,  adapted Streptococcus thermophilus through reductive genome evolution and acquisition of genes conferring proto-cooperation for effective utilization of milk for yoghurt formation in the dairy industry. They detected 56 differentially expressed genes and demonstrated that Lac operon and Leloir pathway genes could be deleted from genomic island in Strep. thermophilus, rendering the acidification of milk. Pastink et al.  performed the comparative functional genomics of amino acid metabolism for glutamate degradation and citrate metabolism for NADPH generation of Lactococus lactis, Lactobacillus plantarum and Streptococus thermophilus LABs starters for industrial fermentations to understand the formation of the flavour at different conditions. Sieuwerts and coworkers in 2009  review and screened the transcription profiling, genome metabolic modelling and experimental evolution of exopolysaccharides produced from Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus for milk fermentation to yoghurt consortia. In a similar report, Adimpong et al.  characterized 33 LABs isolated from African indigenous fermented food products using 16S rRNA gene sequencing and PCR techniques and evaluated the antibiotic activity profile broth microdilution method.
Furthermore, in recent years, biotherapeutics applications of LAB for the prevention and diagnosis of gastrointestinal tract-related disorders have increased. Despite this, many LABs spontaneously produce antibacterial bacteriocins for use in food preservation and probiotics. Targeted delivery using synthetic biology, metabolic engineering, and genomic engineering, on the other hand, would be very fascinating to boost commercial viability . Thus, stable and tunable modification in the LAB host via genome editing may be crucial for the addition of therapeutic compounds to the microbial host. DNA transfer, genome editing and CRISPR-Cas based genome editing have been established for Lactococcus lactis, Lactobacillus reuteri, Lactobacillus gasseri, Lactobacillus casei and Lactobacillus plantarum [192, 214]. With increased food demand and consumption, the involvement of genetic engineering technology has been widely explored to tailor the cellular pathways for desired product [170, 215]. Liu et al.  showed that LAB activity could be improved by four strategies such as adaptive laboratory evolution; combination with meta-omics analysis, system metabolic engineering combined with analysis on metabolic flux regulation in silico model simulation have been investigated to improve food texture, flavour and self-life. Chen et al.  used Adaptive Laboratory Evolution methodology on Lactococcus lactis MG1363 with increased temperature and isolated a mutant TM29 at 39 °C temperature, which resulted from 12% higher lactate production.
Lactic acid bacteria represent a promising biotechnological candidate for surprising health benefits for application in the vegetative food, meat and dairy industries. LAB-based probiotics, bacteriocins, exopolysaccharides, bio preservatives have gained continuous interest with growing food demand and their relevant benefits for human health. LAB fermentation involves the metabolic pathways for the production of LA, ethanol, acetic acid, imparting bio preservatives and promote nutrient enrichments. These products flavours foods, improve functional properties and shelf life by killing harmful pathogens, hence nowadays employed in various food industries for making fermented milk (yoghurt), cheese, beverages, bread, dairy, and meat processing industry. Additionally, LAB based bacteriocins and exopolysaccharides have demonstrated promising contributions in the field of pharmaceuticals, clinical, and diagnostics. Furthermore, the latest advances in metabolic engineering, genome editing, synthetic biotechnological developments may provide a breakthrough for tailoring cellular metabolism for wide LAB industrial application.
Availability of data and material
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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science & ICT) (No. NRF2019M3E6A1103839, NRF-2020R1A2B5B02001757) and supported by Brain Pool Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2020H1D3A1A04081081).
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Raj, T., Chandrasekhar, K., Kumar, A.N. et al. Recent biotechnological trends in lactic acid bacterial fermentation for food processing industries. Syst Microbiol and Biomanuf (2021). https://doi.org/10.1007/s43393-021-00044-w
- Lactic acid bacteria